M734 Fuze - Settings

Settings

A typical mortar firing procedure is for a Squad Leader to select a target and call for one of the four fuze settings. A Gunner sights the mortar onto the target and an Ammunition Bearer sets the fuze. An Assistant Gunner drops the shell into the tube upon a command to fire from the Squad Leader.

Tools are not required to install or set the fuze. It is adjusted by hand, even with Arctic mittens, simply by rotating the top of the fuze clockwise until a 3-letter engraving is above an index line. Additionally, the setting can be changed any number of times without causing damage to the fuze. The four engravings around the circumference of metal housing of the fuze have the following meanings for detonation height:

1. PRX = Proximity air burst between 3 and 13 feet
2. NSB = Near surface burst between 0 and 3 feet
3. IMP = Impact burst on contact. ( In the event an IMP setting fails, detonation is 1/2 seconds after impact. )
4. DLY = Delay after impact of 0.05 seconds in the fuze explosive train before the shell detonates.

In all four settings, the high explosive in the mortar shell is detonated by a cascading explosive train of 4 increasing energies within the fuze. These are the Microdet Electric Detonator, the Explosive Lead, the Explosive Booster, and the Delay Primer Assembly (Figure 3) functioning as follows:

• In the PROX, NSB, and IMP fuze settings, a firing circuit applies a voltage to the small Microdet which faces and ignites a bigger Explosive Lead that channels into the Explosive Booster which initiates the shell's high explosive.
• In the DLY setting, the Explosive Lead is initiated instead by the Delay Primer Assembly, which operates even in the event of power supply or electronics failures.
• Reliability against duds is increased by the fact that if the M734 fails to detonate the mortar shell at one setting, it will immediately and automatically use the next one along, i.e. Failure at the PRX setting causes NSB detonation to be selected. Similarly, failure to detonate at the NSB setting would automatically cause IMP to be selected, and so on.
• This redundancy is a safety factor designed into the fuze to prevent malfunctioning mortar shells from being buried upon ground impact and becoming a risk to civilians after a battle or becoming ammunition for terrorist activities.